As I will show you it has colorful and unusual characteristics as unmistakable as rare, so diagnosis is not difficult. But many more people think they have than have it. In my 50 years of kidney stone prevention I have perhaps a few dozen examples or so, out of many thousands of stone formers.This is another of those long, elaborate articles only the most devoted read.Even so, elaborate as it is, this article tells only part of the story. It simplifies or simply ignores the mechanism for low potassium in dRTA, and left for another time its genetic causes, and also the bone and mineral disorders and treatment outcomes. I forgive myself, as just this part has been most taxing to write and is equally so to read.
In a subsequent article I hope to expand on diagnosis and
treatment, the bone and mineral disorders, genetic transporter disorders, and
take up the novel modern issue of acid retention and its effects on kidneys. So
consider the present article a part of my planned contribution.The featured
illustration of kidney tissue from a patient with dRTA shows many crystal
deposits on a radiograph (panel a), that at surgery mostly are calcified
deposits (panel b) that nearly replace papillary tissue (panels c and d).What
is distal RTA?Kidneys make urine more acid than blood because most of us eat a
diet that imposes an acid load on the body and kidneys need to remove that
acid. But apart from balancing acid excretion to diet acid load, unless kidneys
acidify urine calcium phosphate crystals may form in such profusion as to block
kidney tubules and produce kidney stones.
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This happens because as they conserve water kidneys
concentrate urine calcium phosphate salts far above their levels in blood. If
they simultaneously make urine more acid than blood the calcium and phosphate
will stay in solution and make no crystals. That is the normal state of
affairs.But what would you predict might happen if by magic – bad magic –
kidneys lost their normal ability to make urine acidic? Perhaps not all their
ability, but some significant part of it? Yet withal, retained enough of their
life sustaining glomerular filtration so that their other functions were close
to or in fact normal.
You would predict acid would accumulate in the body and cause
trouble, perhaps leach mineral from bone. Likewise you would predict calcium
phosphate crystals will plug tubules and make stones. This because preservation
of filtration assures delivery of ample calcium into the late nephron and final
urine.You would be right both times.That is the very make and mark of distal
renal tubular acidosis (dRTA).Bad magic.What Does ‘Distal’ Mean?The nephron
begins at the glomerulus. The proximal tubule lies just beyond it. Prior
articles do a reasonable job of showing where things are.
Tubule cells acidify the filtrate in the proximal tubule,
and then again later on, in the collecting ducts. When the latter is defective,
urine itself is invariably too alkaline predisposing to calcium phosphate
crystals and plugs. Such an alkaline urine is common among idiopathic calcium
phosphate stone formers who certainly do not have dRTA. In those who do have
such a defect, stones and crystals are usually far more massive and damaging to
kidneys.When the former is defective (proximal RTA), filtered bicarbonate
cannot be fully reabsorbed as in normals, so more is delivered downstream to
the collecting ducts. There, the cells are competent to secrete acid, but the
amounts of bicarbonate can be so high as to use up all of it and escape into the
urine.
The result is too alkaline a urine. Stones, however, are not
so likely. Serum and therefore filtrate bicarbonate falls, better matching what
remains of proximal reabsorption so the distal delivery slows to a trickle.But,
sometimes a ‘proximal’ problem can indeed cause stones. For example drugs like
topiramate reduce proximal tubule acid secretion and lead to calcium phosphate
stones. In part this may reflect intermittent dosing, so serum bicarbonate
rises between doses, and partly a more acid blood that lowers urine citrate, a
powerful anti – stone molecule.Our concern here is the distal form of
RTA.Clinical Laboratory AppearanceShould we not expect to see high urine pH –
alkaline urine – with acid retention in the blood?
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